The present invention relates to a secondary battery using a non-aqueous electrolytic solution, and particularly to a lithium secondary battery having distinguished charging/discharging characteristics such as a higher voltage, a higher energy density, a higher charging/discharging capacity and a longer cycle life as well as a higher safety.
With miniaturization and weight reduction of portable electronic appliances, development of higher energy density batteries, particularly secondary batteries, has been keenly desired. A lithium secondary battery is now regarded as a promising candidate.
Lithium secondary battery has a high voltage and a high energy density and also a light weight, as compared with a nickel-cadmium battery, a lead storage battery and a nickel-hydrogen battery. However, a lithium secondary battery using lithium metal as a negative electrode-active material has problems of short battery life and poor safety because lithium tends to deposit on the negative electrode surface as dendrites, resulting in occurrence of an internal short-circuit to the positive electrode and inactivation toward the electrolytic solution.
To avoid risks of using lithium metal, lithium secondary batteries using lithium alloys such as Lixe2x80x94Pb, Lixe2x80x94Al, etc. as negative electrode-active materials have been developed. However, even these lithium secondary batteries still suffer from problems of dendrite deposition and pulverization, so that no satisfactory battery life has been obtained yet.
On the other hand, a lithium secondary battery using graphite as a negative electrode-active material has been developed and is now in practical use, where the graphite can store/discharge lithium ions by reaction of diffusing lithium ions into between the c planes of graphite or releasing therefrom, while it is more stable than the chemically active metallic lithium and is free from deposition of lithium dendrites, resulting in prolonged cycle life and increased safety.
In case of using graphite as a negative electrode-active material, the discharge capacity is 370 Ah/kg at most. To increase the capacity of the lithium secondary battery, it is indispensable to use negative electrode-active materials of higher capacity. The negative electrode-active materials of higher capacity include Al, Pb, etc., i.e. elements capable of forming intermetallic compounds with Li, but suffer from a rapid cycle deterioration when used alone or in combination with electroconductive particles as a negative electrode-active material, and thus have not been practically used as a negative electrode-active material.
There are many proposals for using negative electrode-active material comprising particles including an element capable of forming a compound with lithium and a carbonaceous material in a lithium secondary battery (JP-A 5-286763, JP-A 6-279112, JP-A 10-3920). However, since elements having low melting points such as Sn (m.p. 232xc2x0 C.), Pb (m.p. 327xc2x0 C.), Zn (m.p. 419xc2x0 C.), Al (m.p. 660xc2x0 C.), etc. are usable as the element capable of forming a compound with lithium, there is a fear of unexpectedly lowering properties of the products due to aggregation and agglomeration due to melting when carbonization treatment is conducted at 800xc2x0 C. or higher. Further, since elements having higher thermal expansion coefficients such as Sn (22.0 ppm/K at 25xc2x0 C.), Al (23.1 ppm/K at 25xc2x0 C.), Mg (24.8 ppm/K at 25xc2x0 C.), Pb (28.9 ppm/K at 25xc2x0 C.), etc. are usable as the element capable of forming a compound with lithium, there is a fear of failing to maintain adhesiveness to carbon during carbonization treatment and cooling, resulting in incapable of maintaining the particle shape.
Further, Japanese Patent Nos. 2948205 and 2948206 disclose negative electrode materials containing 30 to 90% by weight of silicon. But since silicon is simply mixed with a carbonaceous material and sintered at 600 to 1400xc2x0 C. under non-oxidative atmosphere, uniformity of the quality of the negative electrode materials and improvement of the quality thereof cannot be expected.
An object of the present invention is to provide a lithium secondary battery with a higher capacity and a longer cycle life and a process for producing the same.
According to the present invention there are provided a lithium secondary battery, which comprises a positive electrode, a negative electrode containing a lithium ion-storable/dischargeable negative electrode-active material and a lithium ion conductive, non-aqueous electrolytic solution or polymer electrolyte, the negative electrode-active material containing a carbonaceous material and particles comprising at least one of elements capable of forming a compound with lithium, the elements having a melting point of at least 900xc2x0 C. and a thermal expansion coefficient of not more than 9 ppm/K at room temperature (25xc2x0 C.), the particles being embedded in a plurality of layers of the carbonaceous materials, and the particles together with a carbonaceous material being subjected to a mechanical treatment to make size reduction in advance; a method for using the same in an electric car as a battery assembly; and a process for producing the same.